Control of cell proliferation is important in all multicellular organisms. A number of pathologic processes, including cancer and acquired immunodeficiency syndrome (AIDS) (Ho et al., 1995, Nature 373:123-126; Wei et al., 1995, Nature 373:117-122; Adami et al., 1995, Mutat. Res. 333:29-35), are characterized by failure of the normal regulation of cell turnover. Measurement of the in vivo turnover of cells would therefore have wide applications, if a suitable method were available. Prior to the present invention, direct and indirect techniques for measuring cell proliferation or destruction existed, but both types were flawed.
Direct measurement of cell proliferation generally involves the incorporation of a labeled nucleoside into genomic DNA. Examples include the tritiated thymidine (.sup.3 H-dT) and bromodeoxyuridine (BrdU) methods (Waldman et al., 1991, Modern Pathol. 4:718-722; Gratzner, 1982, Science 218:474-475). These techniques are of limited applicability in humans, however, because of radiation induced DNA damage with the former (Asher et al., 1995, Leukemia and Lymphoma 19:107-119) and toxicities of nucleoside analogues (Rocha et al., 1990, Eur. J. Immunol. 20:1697-1708) with the latter.
Indirect methods have also been used in specific cases. Recent interest in CD4.sup.+ T lymphocyte turnover in AIDS, for example, has been stimulated by indirect estimates of T cell proliferation based on their rate of accumulation in the circulation following initiation of effective anti-retroviral therapy (Ho et al., 1995, Nature 373:123-126; Wei et al., 1995, Nature 373:117-122). Unfortunately, such indirect techniques, which rely on changes in pool size, are not definitive. The increase in the blood T cell pool size may reflect redistribution from other pools to blood rather than true proliferation (Sprent and Tough, 1995, Nature 375:194; Mosier, 1995, Nature 375:193-194). In the absence of direct measurements of cell proliferation, it is not possible to distinguish between these and other (Wolthers et al., 1996, Science 274:1543-1547) alternatives.
Measurement of cell proliferation is of great diagnostic value in diseases such as cancer. The objective of anti-cancer therapies is to reduce tumor cell growth, which can be determined by whether tumor DNA is being synthesized or being broken down. Currently, the efficacy of therapy, whether chemotherapy, immunologic therapy or radiation therapy, is evaluated by indirect and imprecise methods such as apparent size by x-ray of the tumor. Efficacy of therapy and rational selection of combinations of therapies could be most directly determined on the basis of an individual tumor's biosynthetic and catabolic responsiveness to various interventions. The model used for bacterial infections in clinical medicine--culture the organism and determine its sensitivities to antibiotics, then select an antibiotic to which it is sensitive--could then be used for cancer therapy as well. However, current management practices proceed without the ability to determine directly how well the therapeutic agents are working.
A long-standing vision of oncologists is to be able to select chemotherapeutic agents the way antibiotics are chosen--on the basis of measured sensitivity to each drug by the tumor of the patient in question. The ability to measure cancer cell replication would place chemotherapy selection and research on an equal basis as antibiotic selection, with great potential for improved outcomes.
Accordingly, there remains a need for a generally applicable method for measuring cell proliferation that is without hazard and can be applied in the clinical arena.